Objective: To explore the significance of intra-operative motor evoked potentials (MEPs) obtained by monopolar and bipolar stimulation in determining the electrode location giving most pain relief in chronic motor cortex stimulation (MCS).
Methods: Eight patients with chronic pain were implanted with two arrays of four electrodes each. We measured the peak-peak amplitude (Vp-p) of the MEPs recorded intra-operatively with the same stimulus delivered by each single electrode both anodally and cathodally. Electrodes giving the largest MEPs monopolarly were also tested in bipolar stimulation.
Results: In monopolar stimulation the median amplitude of MEPs evoked anodally was 59% larger than cathodally. The mean amplitude of bipolarly evoked MEPs was only 21% and 37%, respectively, of the corresponding monopoles when the poles were 6 mm and more than 8 mm apart. Significant pain relief was obtained in all 5 patients with a cathode used in chronic stimulation being the anode producing the largest MEP. Conversely, in the 3 patients without pain relief, a cathode used in chronic stimulation was the cathode producing the largest MEPs.
Conclusions: In intra-operative testing stimulation should be monopolar because, contrary to bipolar stimulation, the corresponding MEPs are unambiguously related to a single stimulating electrode. The anode providing the largest MEPs should be selected as the cathode in chronic stimulation. Implantable pulse generators allowing monopolar (cathodal and anodal) stimulation for MCS should become available.
Significance: Intra-operative MEP recordings can predict which electrode should be used as the cathode for the best analgesic effect with chronic MCS.